The present invention relates in general to a swash plate type compressor and more particularly to a so-called semi-spherical shoe which is provided between a swash plate of the swash plate type compressor and a piston for reciprocating a piston in response to a rotation of the swash plate, and also relates to a piston joint using the Shoe.
The swash plate type compressor has a swash plate and a reciprocal piston coupled to the swash plate by means of a piston joint. The swash plate is interlocked with and rotated by a rotational shaft.
The piston joint has, for example, a combination of a socket coupled to the piston and a shoe interposed between the socket and the swash plate. The shoe has a generally flat sliding surface which slides along the swash plate and will be called herein under a flat surface. The shoe further has a convex curved surface located on the opposite side of the flat surface. The socket has a concave spherical surface which receives the convex curved surface. The piston joint of the type is disclosed in Japanese Unexamined Publication Nos. 61-135990, 49-65509 and 56-138474.
During operation of the compressor, the shoe exhibits a swinging movement such as a wobble motion and the like relative to the socket of the piston in response to a rotational movement of the swash plate. Accordingly, it is strongly envisaged to provide and maintain a suitable lubricity between the convex spherical surface of the shoe and the concave spherical surface of the socket.
For example, U.S. Pat. No. 4,734,014 teaches to provide a shoe and a socket so that a radius curvature of the convex curved surface of the shoe is smaller than the radius curvature of the concave spherical surface of the socket and an apex or top of the spherical surface is formed flat. This structure will be advantageous since it permits to provide an oil reservoir between the flat surface of the shoe and the concave spherical surface of the socket so that a desired lubricity is obtained. In this structure, the contacting position where the shoe contact the concave spherical surface of the socket is located adjacent to the position of the above-stated oil reservoir and, in other words, the shoe contact the concave spherical surface of the socket at the position of an angular portion which lies on the boundary between the spherical curved surface of the shoe and the flat surface.
In general, some clearances or gaps are provided between the concave spherical surface of the socket and the spherical curved surface of the shoe, and between the flat surface of the shoe and the swash plate and, therefore, these clearances provide, during the operation of the compressor, both a relative vibration in an axial direction of the compressor and a relative vibration in a direction perpendicular to the axial direction. As a consequence of these relative vibrations and the aforementioned swinging movement (or wobble motion), it is assumed that there occurs an unexpected, unusual state that the boundary portion of the shoe contacts the concave spherical surface of the socket at the limited position only which surrounds the oil reservoir and no other place.
If this unexpected state occurs, the reactive force of the compression by the reciprocal movement of the piston is locally integrated to the contacted position, or in other words concentrated on only a part of the contacted position, and therefore it is likely that the conventional shoe structure in which the angular portion contacts the concave spherical surface of the socket as described above results in deformations on the concave spherical surface of the socket, due to plastic deformation, plastic flow and wear. Consequently, the possibility of generation of the relative vibrations described above will be increased.